1. Field of the Invention
The present invention relates to a thermoelectric conversion material to be employed at high temperature for so-called thermoelectric conversion (i.e., direct energy conversion without use of any movable parts), including power generation on the basis of Seebeck effect and electronic freezing on the basis of Peltier effect. The invention also relates to a thermoelectric conversion device containing the material.
2. Background Art
Thermoelectric conversion by use of a thermoelectric conversion material; e.g., thermoelectric power generation or electronic freezing, finds utility in a simplified direct-energy-conversion apparatus having no movable parts that generate vibration, noise, wear, etc.; having a simple, reliable structure; having a long service life; and facilitating maintenance. Thus, thermoelectric conversion is suitable for direct generation of DC power without combustion of a variety of fossil fuels or other sources and for temperature control without use of a cooling medium.
Characteristics of thermoelectric conversion materials are evaluated on the bases of power factor (Q) and figure of merit (Z), which are represented by the following formulas:
Q="sgr"xcex12xe2x80x83xe2x80x83[Formula 1]                    Z        =                              σ            ⁢                          xe2x80x83                        ⁢                          α              2                                κ                                    [Formula  2]            
wherein xcex1 represents Seebeck coefficient; "sgr" represents electric conductivity; and xcexa represents thermal conductivity. Thermoelectric conversion materials are desired to have a high figure of merit (Z); i.e., a high Seebeck coefficient (xcex1), high electric conductivity ("sgr"), and low thermal conductivity (xcexa).
For example, when employed for thermoelectric conversion such as thermoelectric power generation, a thermoelectric conversion material is desired to have a figure of merit as high as Z=3xc3x9710xe2x88x923 1/K or higher and to operate without variation for a long period of time under varying operation conditions. Mass production of thermoelectric power generators for use in vehicles or employing discharged heat gives rise to demand for a thermoelectric conversion material which has sufficiently high heat resistance and strength, particularly at high temperature, and resistance to deterioration in characteristics, as well as a method for producing the material at high efficiency and low cost.
Conventionally, PbTe or silicide materials including silicide compounds such as MSi2 (M: Cr, Mn, Fe, or Co) and mixtures thereof have been used to serve as the aforementioned thermoelectric conversion materials.
Sb compounds such as TSb3 (T: Co, Ir, or Ru) have also been used. For example, there has been disclosed a thermoelectric material which comprises a material containing CoSb3 as a predominant component and an impurity added for determination of conduction type (L. D. Dudkin and N. Kh. Abriko Sov, Soviet Physics Solid State Physics (1959) p. 126; B. N. Zobrinaand, L. D. Dudkin, Soviet Physics Solid State Physics (1960) p. 1668; and K. Matsubara, T. Iyanaga, T. Tsubouchi, K. Kishimoto, and T. Koyanagi, American Institute of Physics (1995) p. 226-229).
Thermoelectric conversion materials formed of PbTe exhibit a high figure of merit (Z)xe2x80x94an index of thermoelectric propertiesxe2x80x94of approximately 1xc3x9710xe2x88x923 1/K at about 400xc2x0 C. However, the materials have a low melting point and poor chemical stability attributed to Te, a volatile component contained in the materials, and cannot be used at high temperature of 500xc2x0 C. or higher. In addition, since cumbersome production steps are required due to presence of a volatile Te component in the materials, variation in product characteristics tends to be caused by variation in composition of the materials, failing to attain effective mass-production. Another disadvantage is that the raw materials for producing the thermoelectric conversion materials are expensive and highly toxic.
Silicide materials including silicide compounds such as MSi2 (M: Cr, Mn, Fe, or Co) and mixtures thereof can be produced from inexpensive raw materials; contain no toxic components; are chemically stable; and can be used at temperatures of about 800xc2x0 C. xe2x80x9cNetsuden Handotai To Sono Oyo,xe2x80x9d authored by Kunio NISHIDA and Kin-ichi UEMURA, (1983) p. 176-180 (published by Nikkan Kogyo Shimbun) discloses a comparatively inexpensive method of producing these silicide materials. However, these silicide materials exhibit a thermoelectric property (e.g., a figure of merit (Z) of approximately 1-2xc3x9710xe2x88x924 1/K) of about one-tenth that of PbTe and cannot provide sufficient thermoelectric properties comparable to those of PbTe.
Thermoelectric materials containing an Sb compound such as TSb3 (T: Co, Ir, or Ru) as a predominant component (e.g., CoSb3) are produced from non-toxic, comparatively inexpensive raw materials and are known to exhibit a comparatively high figure of merit ( less than 1xc3x9710xe2x88x923 1/K).
In the production of a conventionally known thermoelectric conversion material having a chemical composition of CoSb3, it is desirable that cubic CoSb3 crystal phase is exclusively formed in the material to serve as a constitutional crystal phase, with other crystal phases (CoSb, CoSb2, and Sb), which are detrimental to thermoelectric properties, being removed. However, in reality, when a production method involving melting CoSb3 is employed, undesired phases (CoSb, CoSb2, and Sb) other than CoSb3 phase are precipitated during the course of solidification. In order to generate a crystal phase formed only of CoSb3 from such a molten material, heat treatment at approximately 600xc2x0 C. for about 200 hours is required, and such treatment disadvantageously prolongs the time required for production steps.
In addition, when a production method in which a solidified CoSb3 melt is pulverized and sintered is applied, undesired phases (CoSb, CoSb2) precipitated during solidification and having a higher density than that of CoSb3 are transformed into CoSb3 phase during firing. This phase transformation causes volume expansion, thereby disadvantageously inhibiting sintering. Specifically, sufficiently densified material has never been produced, even when pulverized CoSb3 is hot-pressed at 5xc3x97103 kg/cm2 and 600xc2x0 C. (Reference: K. Matsubara, T. Iyanaga, T. Tsubouchi, K. Kishimoto, and T. Koyanagi, American Institute of Physics (1995) p. 226-229). The maximum density of the thus-sintered CoSb3, reported in the reference, is 5.25 g/cm3, whereas the theoretical density of cubic CoSb3 is 7.64 g/cm3. Thus, the sintered CoSb3 is a considerably fragile material, and has poor strength at high temperature.
In order to attain satisfactory durability of a material formed of heavy elements such as Bi, Te, Se, and Pb against contact with industrial process discharge gas and to prevent vaporization of constitutional components in a high-temperature reaction atmosphere and contamination with the vaporized components, there has been desired a new material which can be produced at low cost; causes less environmental pollution; and can be used without causing variation even at high temperature.
In view of the foregoing, a strong tendency to use an oxide as a thermoelectric material has rapidly arisen. Generally, an oxide has low mobility and a typical carrier concentration of about 1019 cmxe2x88x923, exhibiting no conductivity, unlike a metal. Thus, it has been commonly accepted in the art that an oxide cannot serve as thermoelectric conversion material. However, in 1997, an oxide of layer structure, NaCo2O4, was surprisingly found to exhibit strong thermoelectromotive force despite its low resistivity (Japanese Patent Application Laid-Open (kokai) No. 2000-211971). Thermoelectric properties of this class of oxide are remarkably superior to those of other oxides, and approach those of existing thermoelectric material used in practice.
However, this oxide also has a drawback in that thermoelectric properties of products vary greatly in accordance with production conditions, due to sublimation of Na during sintering. In addition, when this oxide is used at high temperature, sublimation of Na disadvantageously deteriorates thermoelectric properties, and when the oxide is allowed to stand in air, resistivity problematically increases. Furthermore, Na is highly reactive to water contained in air, and the resultant reaction may deteriorate performance of products.
The present inventors have carried out extensive studies in order to solve the aforementioned problems, and have found a novel oxide thermoelectric conversion material. The present invention has been accomplished on the basis of this finding.
Thus, an object of the present invention is to provide a thermoelectric conversion material which has low toxicity and can be used at high temperature of 500xc2x0 C. or higher without variation in performance. Another object of the present invention is to provide a thermoelectric conversion device containing the material.
Accordingly, in one aspect of the present invention, there is provided a thermoelectric conversion material formed of an oxide represented by (Ca3-xMx)Co4O9 (M: Sr or Ba, 1.2 greater than x greater than 0.5).
The oxide may be oriented along the C axis.
In another aspect of the invention, there is provided a thermoelectric conversion device containing the thermoelectric conversion material.
In still another aspect of the invention, there is provided a method of thermoelectric conversion comprising effecting thermoelectric conversion by use of a thermoelectric conversion material.